The invention relates to a process and a device for drawing off and blocking off a melt, in particular of plastic material, with a heatable melter, which is provided with a blocking valve and acts as a melt distributor and from which the melt flows into a nozzle plate, which divides the melt by means of nozzles into a plurality of strands, whereby the temperature of the melter and nozzle plate is regulated separately and the blocking valve is closed to block off the melt, and the nozzle plate is tightly closed off by a cover.
It is necessary to seal off the melt at the nozzle apertures if the melt flow is blocked off, in order to prevent the air from penetrating from the nozzle apertures; said air can trigger an oxidation process of the melt, which leads to an undesired chemical change in the melt material. To date the cover used for this closure was usually screwed to the nozzle plate. It is thereby necessary that the cover rests with significant pressure on the nozzle plate, in order to obtain the requisite sealing effect. It has turned out that apparently owing to the existing temperature differential a distortion of the cover is inevitable, a feature that results in the nozzle apertures being sealed only incompletely and thus the air having access.
The object of the invention is to improve the seal of the nozzle apertures in the case of the melt flow being blocked off. According to invention this problem is solved in that to draw off the melt with the blocking valve open, the temperature of the melter and the nozzle plate is kept substantially at the same level, and to block off the melt with the blocking valve closed the nozzle plate is lowered to a temperature just above the solidification temperature, and with the closure of the blocking valve the cover cooled to a temperature considerably below the solidification temperature is laid on the nozzle plate into which a residual melt flows and seals off the nozzle apertures in the solidified state.
The temperature in the melter and the nozzle plate that is kept at the same level while drawing off the melt is changed in such a manner when blocking off the melt flow that, while the temperature in the melter is kept unmodified, the temperature level in the nozzle plate is lowered, and in particular so far that in the nozzle plate a temperature just above the solidification temperature of the melt material prevails. If then owing to the considerable cooling of the cover, the cover accommodating the melt material converts the melt material into the solidified state, the solidified material can seal off the nozzle apertures, and in particular also permanently, since owing to the lowering of the temperature in the nozzle plate, the solidified melt material held by the cover cannot be melted again from that direction. The result is thus, owing to the solidified melt material a reliable sealing of the nozzle apertures, thus reliably preventing the air from penetrating and thus oxygen into the region of the melt. This kind of sealing of the nozzle plate also allows the cover to be laid with less precision on the nozzle plate, without requiring a specific pressure to this end, since the necessary seal is brought about not by the cover itself but, as said, by the solidified melt material.
The device to effect the process described above is based on a heatable melter, which is provided with a blocking valve and which is acting as a melt distributor and to which is attached a nozzle plate, which divides the melt into a plurality of strands by means of nozzles and which can be sealed off with a cover. This device is designed in such an advantageous manner that the nozzle plate and the cover are provided with one separate and individually controllable heating or cooling system each, whereby between the melter and the nozzle plate a temperature insulating layer, which connects sealingly melter and nozzle plate, is installed, and the cover exhibits a recess, extending over the nozzle apertures, as the collecting basin for the melt cooled by the cover.
The recess in the cover, which serves as the collecting basin for the melt cooled by the cover, can be relatively flat, so that a relatively thin layer is formed by the solidified melt material in the recess. This is totally adequate for the sealing effect. In so doing, the temperature insulating layer provided between melter and nozzle plate provides that the temperature prevailing in the melter has no effect on the temperature in the nozzle plate that is cooled in contrast.
If the melt is to be drawn off again from the device, the cover is removed from the nozzle plate. Depending on the melt material used, the cover can take along in its recess the solidified melt layer. In this case the cover can be provided with a ram penetrating the cover in the region of its recess, in order to push out the solidified melt material. With the cover removed, the ram is pushed forward, whereby the solidified melt layer drops out of the recess of the cover.
The cover can be affixed or removed by hand, but it is also possible to carry out this procedure, e.g., with a swivel or slide mechanism. In so doing, it is especially advantageous that the cover does not constitute a risk for the operating personnel on account of its cooling.